This invention relates generally to water distillation and more specifically to an improvement in a distillation apparatus in which successive stages of redistillation are provided, so as to achieve a greater degree of purity.
Distillation of water or other liquids through the process of evaporation followed by condensation of the vapor is a well-known art, known to mankind for centuries. Typically when it is desired to separate a liquid from suspended impurities or from components having considerably higher boiling points the procedure cells for raising the temperature of the feed liquid, vaporizing the liquid in question, while leaving the undesired impurities behind in their solid or liquid states, and then condensing the now-pure vapor and collecting the condensate. However, despite sophisticated distillation apparatuses, a problem still exists in achieving advanced levels of purity when a particular class of contaminants is involved. In some sensitive applications just a few parts per million of these contaminants remaining in a distillate will be enough to produce significant problems. A particularly troublesome problem exists when trying to separate a liquid from volatile impurities having boiling points close to its own boiling point. Silicon hexafluoride is an example of such a substance having a boiling point close to the 212.degree. F. boiling point of water, and therefore significant reduction of this contaminant is extremely difficult to achieve using conventional stills. Once a typical distillation apparatus has attained a steady-state operating condition, the amount of volatile impurities which also have been boiled off from the feed liquid begin to build up and saturate the ambient atmosphere within the apparatus. Once the ambient atmosphere has become saturated and an equilibrium condition has been established between the condensates and the volatile impurities in the ambient atmosphere, not only will the efficiency of the apparatus become impaired but the absolute degree of purity achievable by the apparatus will become limited. U.S. Pat. No. 3,975,241 attempted to increase the degree of purity achievable by conventional stills by introducing a reboiler located within the interior of the still which would collect the condensate and reboil it, thereby boiling off additional amounts of impurities. However, the efficacy of this modification was somewhat limited because by being located within the interior of the still, the ambient atmosphere surrounding the distillate was in communication with the main ambient atmosphere of the still itself and therefore an equilibrium condition quickly arose between the two atmospheres which limited the amounts of volatile impurities that could be boiled off from the distillate. Because of the saturated condition of the ambient atmosphere the undesirable vapors would tend to condense almost immediately after being evaporated and fall back into the distillate, thereby recontaminating it. Although the main ambient atmosphere of the still was vented to a certain degree to the outside world, the essentially closed system within the still limited the ultimate reduction in trapped volatile impurities achievable by the reboiling process.
It is an object of this invention to enhance the ability of distillation apparatuses to achieve significant reduction of the amount of volatile components within a distillate, particularly of those components with boiling points close to the boiling point of the distillate, and to do so in a simple and relatively inexpensive manner.
It is a further object of this invention to minimize the likelihood of vaporized impurities recontaminating a purified distillate.
It is still a further object of this invention to provide additional stages of reboiling in a manner which may be readily adaptable to existing stills without significant structural or mechanical modifications thereto.